Local honey - be sure to buy Kefalonian wild thyme honey, it really does taste special - ,and the local wine, Robola.
There are any number of tacky gifts to be had, though to be fair most of the tourist shops have remained reasonably tasteful and low-key. Souvenirs aplenty, as you'd expect. Some of the jewellery is of reasonable quality and price - you are pretty unlikely to get ripped off on Kefalonia and the Greeks are generally keen to see you get what you pay for in any transaction.
